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Provenance for a supernova

Like an antiques dealer, astrophysicists often are faced with the challenge of estimating the age of an artifact such as a supernova remnant. Calculations can yield reasonably good estimates, but because most art happened long before modern instruments, the estimates have margins of error.

Like the antiques business, the most valuable artifacts are the ones with a provenance, a record that removes all doubt about its origins and history. The Crab Nebula has a provenance, starting with records kept by royal court astronomers in China and Native Americans.

The Crab appeared in July or August A.D. 1054, according to Chinese records, probably on July 5, according to Native American cave drawings White Mesa and Navajo Canyon. Appearing in the sky above the southern horn of the constellation Taurus was a star the Chinese described as six times brighter than Venus, about as brilliant as the full Moon - and visible during the day for almost a month, and at night for a year.

Small wonder. At its peak it blazed with the light of about 400 million suns. That was enough energy to have destroyed all living things on any planet within 50 light years. Fortunately for us, the Crab is more than 7,000 light years away, so the pulse Earth received was about 1/20,000th what it would have been for a closer world.

Then it faded from view and memory until 1731 when English physicist and amateur astronomer John Bevis observed the strings of gas and dust that form the nebula. While hunting for comets in 1758, Charles Messier spotted the nebula, spotted it as he started his list of objects that are not comets, his real quarry. The nebula became M1 in his famous "Catalogue of Nebulae and Star Clusters," published in 1774. Lord Rosse named the nebula the "Crab" in 1844 because its tentacle-like structure resembled the legs of the crustacean.

In the decades following Lord Rosse's work, astronomers continued to study the Crab because of their fascination for the strange object. In 1939, astronomer John Duncan concluded that the nebula was expanding and probably originated from a point source about 766 years earlier (he was only off by a century, a remarkably accurate estimate). Historians later linked the Crab with the "guest star" of 1054.

Walter Baade probed deeper into the nebula, observing in 1942 that a prominent star near the nebula's center might be related to its origin. Six years later, scientists discovered that the Crab was emitting among the strongest radio waves of any celestial object. Baade noticed in 1954 that the Crab possessed powerful magnetic fields, and in 1963, a high-altitude rocket detected X-ray energy from the nebula.

Radio waves. X-rays. Strong magnetic fields. Scientists knew that the Crab Nebula was a powerful source of radiation, but what was its power source? They discovered it in 1968: an object in the nebula's center - Baade's prominent star - that emitted bursts of radio waves 30 times per second. Scientists soon concluded that the pulsar was a neutron star because theory suggested that these stars existed at the centers of supernova remnants.

The Crab Pulsar acts as a celestial power station, generating enough energy to keep the entire nebula radiating over almost the whole electromagnetic spectrum. Because of the pulsar's power, the nebula shines brighter than 75,000 suns. That's bright enough to draw the constant attention of astrophysicists from across the planet and the spectrum.

Cat on a hot Crab

In 1996, astronomers using NASA's Hubble Space Telescope found that the Crab is even more dynamic than previously understood after they assembled a cosmic "movie" from a series of Hubble observations showing that the interior of the nebula "changes its stripes" every few days.

"We took the images a few weeks apart because we knew that it might be possible to observe slight differences in the Crab over a short time," said Dr. Jeff Hester of Arizona State University in Tempe, AZ. "But I don't think that any of us were prepared for what we saw."

The Hubble team found that material doesn't move away from the pulsar in all directions, but instead is concentrated into two polar "jets" and a wind moving out from the star's equator. The most dynamical feature is the point where one of the polar jets runs into the surrounding material forming a shock front. The shape and position of this feature shifts about so rapidly that the astronomers describe it as a "dancing sprite," or "a cat on a hot plate." The equatorial wind appears as a series of wisp-like features that steepen, brightenand then fade as they move away from the pulsar to well out into the main body of the nebula.

"Watching the wisps move outward through the nebula is a lot like watching waves crashing on the beach - except that in the Crab the waves are a light-year long and are moving through space at half the speed of light," said Hester. "You don't learn about ocean waves by staring at a snapshot. By their nature waves on the ocean are ever changing. You learn about ocean waves by sitting on the beach and watching as they roll ashore. This Hubble 'movie' of the Crab is so significant because for the first time we are watching as these 'waves' from the Crab come rolling in."

How to cook a crab

Stars the mass of our sun eventually die quietly, becoming white dwarfs shining for eons with leftover heat. If a star has about 10 times the Sun's mass, the extra internal pressure burns nuclear ash into heavier elements until it fuses silicon into iron and nickel. Further fusion absorbs rather than yields energy, so the furnace flips off, the star collapses and then rebounds in a massive blast that spews hot gas - including virtually every element in the periodic table - into space.

Such a supernova created the Crab Nebula.

Besides the interstellar debris, supernova explosions often leave behind a cinder, a dense, collapsed core created by the compression of electrons and protons. Called a neutron star, the object is about 20 km (12 mi) wide, has a mass greater than our Sun, and a density equivalent to cramming a World War II battleship into the head of a pin.

The star's gravitational field is about 300,000 times stronger than the Earth's. Its rotation also increases dramatically during the collapse, like a skater spinning faster as she retracts her arms. The neutron star in the Crab Nebula rotates 30 times per second or 3.4 million miles per hour at its equator.

Some neutron stars - such as the Crab - emit radio waves, light, and other forms of radiation that appear to pulse on and off like a lighthouse beacon as the pot point towards then away from Earth. The real difference between a neutron star and a pulsar is that a pulsar has a magnetic field that is misaligned with the rotation axis -- being tilted at an angle of about 30 degrees to the rotation poles.

Their rapid rotation makes them powerful electric generators, capable of accelerating charged particles to energies of millions of volts, lighting up the nebula around it. (The irregular structure comes from the accelerating gas slamming into slower, colder gas in interstellar space). This drags on the pulsar, so that it spins slower over time. It will take about 10,000 years for the pulsar to slow to half its current rotation speed. The Crab's pulses will weaken, and its X-ray emissions eventually will end. The nebula itself will disappear after only a few thousands years. Eventually only the radio pulsar, beaming every few seconds, will remain.

Plagiarised from : http://science.nasa.gov/newhome/headlines/ast28sep99_1.htm